Since reading various threads about Gatwick and the ELL, I've started to
wonder - has anyone considered combining them with a service from
Gatwick Airport to Liverpool Street (stopping all stations via
Forest Hill)? The idea might seem strange at first, but it does appear
to have a lot going for it - giving Surrey and South London direct
access to Liverpool Street station and E1 would greatly increase
connectivity, and for many airport passengers with heavy luggage, the
convenience would more than make up for the extra journey time (though
obviously those in a hurry would continue to use Thameslink).

I think the demand for such a service would be high, but have I
overestimated it? And if demand were high, would they be able to run
enough trains to accommodate it?

The obvious problem with such a route is the infrastructure
requirements. But what exactly would such a service require? I can think
of the following things:
Restoration of the Shoreditch - Liverpool Street connection, providing
platform capacity at Liverpool Street (Crossrail could do that), extra
platforms at Gatwick and/or grade separation N of Gatwick (since the
existing arrangements are clearly inadequate), conversion of the ELL to
3rd rail (as is already planned UIVMM) and probably a signalling
upgrade.

Can anyone think of anything else that would need to be done?